Answer guidance is not shown when answering

[cellio]

We have a site setting for Answer Guidance, and a per-category override. The corresponding Question Guidance is shown on the "new post" page in the green box at the top. Answer guidance is not shown when you answer a question. It is shown when you edit an answer, but that's late -- guidance is most useful when you're starting to answer. We don't want to be noisy for experienced users, but we should find a way to show it earlier.

I can think of three approaches we could take:

1. Show it statically above the form (like we do for questions and when editing an answer). But that means it would show up on every (non-closed) question you view, which feels tedious and could lead to "banner blindness".
2. Show it (somehow, non-disruptively) when you start to type an answer.
3. Don't show it by default but add some UI control that makes it very obvious that answer guidance exists, and that control toggles it.

1 and 2 could be annoying especially for experienced users (adding extra stuff to the page that's "in the way", especially bad on phones), so if we do that, we should have a way to collapse the guidance and remember that state. (Collapsing is better than removing entirely because in the future you might need to look at, say, some formatting instructions that were in there.)

3 is a minimal solution that doesn't require us to be clever or remember state. 3 is probably easy; I don't know how hard 1 and 2 are.